Offshore Wind RD&D: Large Offshore Rotor Development

Overview

Sandia National Laboratories Wind Energy Technologies Department, creates and evaluates innovative large blade concepts for horizontal axis wind turbines to promote designs that are more efficient aerodynamically, structurally, and economically. Recent work has focused on the development of a 100-meter blade for a 13.2 MW horizontal axis wind turbine, a blade which is significantly longer than the largest commercial blades that existed at the beginning of this project (approximately 60 meters long). The table provides a summary of four design studies that were performed for 100-meter blades:

All-glass Baseline Blade:

SNL100-00

114 ton weight

SAND2011-3779

Carbon Design Studies:

SNL100-01

74 ton weight

SAND2013-1178

Advanced Core Material:

SNL100-02

59 ton weight

SAND2013-10162

Advanced Geometry/Flatbacks:

SNL100-03

49 ton weight

SAND2014-18129

Detailed design models (Sandia NuMAD format) for each of the four blade designs can be downloaded below. A 13.2 MW turbine model (NREL FAST format) is also available for download.

Design loads analysis of the baseline model based on a critical subset of international standards demonstrated acceptance of the design with respect to strength, fatigue, deflection, and buckling. Challenges and opportunities for large blade research are summarized below in the referenced reports and linked documents.

These 100-meter blade models can provide a starting point for consideration of blade innovations by the research community with potential performance improvement, weight reduction, and cost improvements. A design scorecard is provided below for use by researchers to compare the effect of innovations on the principal design drivers which include weight, fatigue life, buckling, tip deflection, maximum strains, and aeroelastic stability (flutter margin). The 13.2 MW turbine models also provide a starting point for turbine and turbine control studies.

Aeroelastic stability (flutter) and aeroelastic performance of large blades were discussed in References 2, 4, and 5.

100-m Carbon Blade Design Studies: SNL100-01
An updated 100-m blade reference model was developed, termed as SNL100-01 (references 6-9). The new design was a modification to the baseline SNL100-00 design with the same external geometry; however, a carbon spar cap was introduced into the design. The weight of the SNL100-01 design is about 74,000 kg, which is a 35% reduction in weight from the baseline all-glass blade. A series of carbon spar cap designs were analyzed and documented with SNL100-01 as the final design model. In addition to the new spar cap, additional associated modifications were made including reduction in spar and TE reinforcement width, movement of the two principal spar caps, and thinning of the root.

References 10 and 11 describe the SNL Blade Manufacturing Cost Model (version 1.0) that was developed under this project.

SNL100-02: Advanced Core Material and New Core Strategy
New core materials were evaluated to produce the SNL100-02 design (Reference 12). Various foams, balsa, and structured (engineered) core identified in an industry survey of core materials, were evaluated along with a new core material strategy in a series of structural design studies. The new strategy utilized balsa in critical buckling areas (Trailing Edge Panel: Inboard) and PET foam in the non-critical buckling areas (Trailing Edge Panel: Outboard), as shown in the figure. In addition to the weight reduction achieved, a secondary benefit was found in that these core materials are regrowable (in the case of balsa) and recyclable (in the case of PET foam).

SNL100-03: Flatback Airfoils and Blade Slenderness Study
The effects of flatback airfoils were evaluated in a fourth and final series of design studies. The advantages and disadvantages of high blade slenderness low blade solidity were quantified with respect to tip deflection, flap-wise & edge-wise fatigue resistance, panel buckling capacity, flutter speed, manufacturing labor content, blade total weight, and aerodynamic design load magnitude (References 13 and 14).

Challenges and Opportunities

Initial ObservationsA 100-m blade using conventional geometry and all-glass materials is possible. All design requirements are satisfied including maximum strains, tip-tower clearance, buckling resistance, and fatigue life. However, the blade weight for the initial SNL100-00 design was very high (and not cost-effective). The subsequent SNL100-01, SNL100-02, and SNL100-03 blade studies demonstrated a pathway for weight reduction, as shown in the figure where projections of weight growth are compared with commercial blades and research concept blades (including the SNL100-XX series).

Current and Future Work: Potential Research Directions
Significant opportunity exists to reduce weight and cost through innovations and structural optimization. These reference models provide a starting point for block and turbine design and analysis studies. In design studies, carbon fiber, very thick airfoils such as flatback airfoils, bend-twist coupling, geometric sweep, pre-bending, and unique architecture, anti-buckling devices, structured core, and active control could be considered. Many of these approaches have been explored in these SNL100-XX design studies.

Other considerations for future work and potential research in large rotor technology are outlined in the SNL100-00 design report (Reference 1) provided below, Reference 7, and the project final design report for SNL100-03 (Reference 14).

Sandia Blade and Turbine Design Models: Reports and Model Files

Large Offshore Rotor Model Download

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